What exactly is the difference here.
We learned more about Dante than just "his mother was killed by demons". The entire narration about Sparda and the war "2 millenniums ago" is backstory. The dialogue between Dante and Trish about how his mother ["and brother"] were killed by demons, that his father is Sparda, that he works as a private eye that takes any job, and also that he killed each demon he encountered until he found the right one to lead him to Mundus or get his attention, is backstory. All of that happened before we encountered the character, we're just hearing about it. Then every other game until recently was being carried by "Sparda did a thing, now Dante has to do a thing" to get people to care about why Dante is there and doing a thing, and 5 got carried off of Dante and Vergil ending a sibling rivalry that persisted since they were eight years old that no one in their right mind in- and out-of-universe should even care about anymore because people in their 40s still caring about stuff they did below the age of 10 is inherently stupid.
Given all that, what backstory Nero ended up having in 4 ("working for the Order, likes Kyrie, brother-of-sorts to Credo, ostensibly related to Sparda, lost the use of his arm but regained it with a mutation to save Kyrie") should also work in terms of how minimal it is, except it doesn't really, but pinning that on "overreliance on backstory" when the series has done the same thing around Dante for about four-and-a-half games now and supposed plot fixes in this thread for Nero involve him "learning about his identity" (thus learning the backstory that put him in that path because he doesn't know it, suggesting the character needs to clear that to move forward) or even playing as an even younger version of Dante to experience the backstory of Fortuna in a glorified protracted flashback happening before even the prequel game we played is useless. It's actually less than useless, it's confused.